Abraham warned that the technology and know-how Russia is providing to Iran could be used for destructive purposes.

"We have long been concerned that Iran's only interest in nuclear civil power, given its vast domestic energy resources, is to support its nuclear weapons program," the energy secretary said.

Russia says the nuclear power plants are strictly for civilian use and that international inspectors who have visited the facility several times have found no violations of international regulations.

The United States is also concerned that Iran may also be using its Russian contacts to push forward other types of weapons.

"We do have concerns about the potential assistance Iran is gaining from contacts with Russia with respect to chemical and biological weapons," U.S. Ambassador to Moscow Alexander Vershbow said during Thursday's news conference.

He added that Washington was also concerned with the possibility of "transfers of advanced conventional weapons" to Iran. "All these concerns are under discussion right now," Vershbow said.

Protesters in Tehran during a recent anti-U.S. demonstration

In May, during a visit to Moscow by Bush, Russian President Vladimir Putin denied U.S. charges that Russia's nuclear technology co-operation with Iran could help Tehran to obtain weapons of mass destruction.

Putin and Bush had met to discuss a landmark deal to cut their strategic nuclear forces by roughly two-thirds over 10 years.

But Bush urged Putin to stop providing weapons technology to Iran, warning the Russian leader: "If you arm Iran, you're liable to have the weapons pointed at you."

A Bush aide at the time described Russian help to build the Bushehr plant which as "the single most important proliferation threat there is."

Two weeks ago, tens of thousands of Iranians chanting "Death to America, death to George Bush" took to the streets of Tehran in an demonstration against what they view as U.S. interference in Iran's affairs.

Supreme Leader Ayatollah Ali Khamenei called the rally to show popular opposition to Bush's statement days earlier voicing support for reform in Iran.

CNN's Jill Dougherty says the matter of nuclear cooperation is a major irritant in the otherwise improving relationship between Moscow and Washington.